Non-contact temperature measurement instruments allow a user to measure the temperature of a surface of an object or portion of an object without touching the object. One common type of non-contact temperature measurement device is an infrared thermometer. Infrared thermometers determine the temperature of an object by measuring the infrared radiation emitted by the object. The amount of infrared radiation that is emitted at particular wavelengths is correlated with the temperature of the object. If the amount of infrared energy emitted by the object and its emissivity are known, then the object's temperature can be determined without contacting the object. The optical system of an infrared thermometer collects the infrared energy from a measurement spot and focuses it upon a detector. The detector then converts the energy to an electrical signal that can be displayed in units of temperature. Many infrared thermometers allow a user to ascertain the temperature of a spot on an object by aiming the thermometer at the object.
As discussed above, many infrared thermometers work by using a known relationship between thermal radiation and emissivity. The emissivity of a material is the ratio of energy radiated by a particular material to energy radiated by a blackbody at the same temperature. Emissivity is a measure of a material's ability to radiate absorbed energy. A true blackbody has an emissivity of one, while all real objects have an emissivity that is less than one. The output signal of the detector of an infrared thermometer is related to the thermal radiation, j*, of the target object through the Stefan-Boltzmann law:j*=εσT4 
The constant of proportionality, σ, is the Stefan-Boltzmann constant, and ε is the emissivity of the object. An infrared thermometer measures the thermal radiation j* of the object. Thus, if the emissivity of the target object is known, the Stefan-Boltzmann law can be used to calculate an approximate temperature, T, of a spot on the surface of the object.